The present invention relates to a Zinc Sulfide activated with Silver (ZnS:Ag) based fluorescent material for detecting particle beams which enables to detect particle beams such as alpha rays efficiently, and also relates to manufacturing method thereof. More specifically, the present invention relates to Zinc Sulfide activated with Silver (ZnS:Ag) based fluorescent material for detecting particle beams which attains to have a low sensitivity to gamma rays (electron beams) exiting as background in detecting particle beams such as alpha rays in order to reduce the decay time for measurement with a high detection efficiency and to reduce the quantity of afterglow which may disturb the measurement with a high detection efficiency, and also relates to manufacturing method thereof.
The Zinc Sulfide activated with Silver (ZnS:Ag) based fluorescent material represented by P11 fluorescent material conventionally and commercially available has such characteristics as having a lower sensitivity to gamma rays existing as background in comparison with other types of scintillators, and also yielding a larger quantity of fluorescence in response to particle beams such as alpha ray.
ZnS:Ag based fluorescent material has been used as the fluorescent material for neutron scintillators for detecting alpha rays conventionally or since 1950's in order to detect neutrons by way of detecting 3He and alpha rays emitted by nuclear reactions between neutron and 6Li used as the neutron converter as inn+6Li→3He+α, orin order to detect neutrons by way of detecting 7Li and alpha rays emitted by nuclear reaction between neutron and 10B used as the neutron converter as inn+10B→7Li+α.
As ZnS:Ag based fluorescent material represented by P11 fluorescent material has such a disadvantage as providing a larger quantity of afterglow, it is difficult with this type of fluorescent material to realize measurements with a high detection efficiency. In addition, ZnS:Ag based fluorescent material has generically a gamma-ray sensitivity, which yields a background in detecting particle beams such as alpha ray and detecting neutrons through neutron converters.
In Patent Literature 1 “Particle Ray Detector and Neutron Detector using ZnS based Fluorescent Material”, irradiation experiments for confirming that the fluorescence spectrum in alpha-ray irradiation is different from the fluorescence spectrum in gamma-ray irradiation to ZnS:Ag, Cl based fluorescent materials conventionally manufactured were performed by using 241Am as a radiation source for alpha rays, and using another radiation source for 60 keV gamma rays by covering the surface of the radiation source for alpha rays with a thin plate in order to remove the effect of alpha rays Note that Patent Literature 1 uses the term of “ZnS:Ag, Cl based fluorescent material” in order to refer to the fluorescent material, and that P11 fluorescent material, etc. used generally in commercial products, is labelled simply as ZnS:Ag. This difference in labelling with or without “Cl” comes from the fact that Sodium Chloride (NaCl) is used generally as a flux in fabricating ZnS:Ag based fluorescent materials, and thus “Cl” is added in case of labelling more precisely. Patent Literature 1 discloses a fluorescence spectrum diagram showing that the fluorescence spectrum obtained by alpha-ray irradiation is different from the fluorescence spectrum obtained by gamma-ray irradiation, which is incorporated in FIG. 1 in the present invention as reference. It is proved in FIG. 1 that the spectral intensity of the fluorescence spectrum increases globally at the short wavelength range at 420 nm or below 420 nm.
FIG. 2 shows alpha-ray irradiation fluorescence spectra for 1109-041 ZnS:Ag—Cl fluorescent material manufactured by Nichia Corporation and P11 ZnS:Ag fluorescent material commercially available in market. As understood from FIG. 2, it can be confirmed that both spectra show an identical fluorescence spectrum and those fluorescent materials have identical characteristics.
In Patent Literature 1, the sensitivity to gamma rays may be reduced by detecting only the wavelength range at 420 nm or shorter in the fluorescence spectrum shown in FIG. 1 by using the optical filter in order to reduce the detected intensity of gamma ray, and the decay time may be reduced by reducing the effect of afterglow.
[Patent Literature 1] JP 2005-300479 A